**2. Materials and Methods**

The coatings were deposited by magnetron sputtering in the direct current mode. The sputtered composite target TaZrSiB—with element composition of 70.8 wt.% Ta, 18.6 wt.% Si, 7.4 wt.% Zr, and 2.9 wt.% B, and phase composition of TaSi2-Ta3B4-(Ta,Zr)B2, 120 mm diameter, and 6 mm thickness—was produced by hot pressing on DSP-515 SA installation ("Dr. Fritsch", Germany). The feedstock powders for hot pressing were fabricated by the ball-milling of the combustion products of a reactive mixture of Ta, Zr, Si, and B. The details of the combustion synthesis protocol can be found elsewhere [31].

VK-100-1 grade alumina plates (99.7% Al2O3, JSC Policor, Kineshma, Ivanovo region, Russia), and alumina discs VOK (VNIITS, Russia) and VT-1-0-grade (OAO Conmet, Moscow, Russia) were used as the substrates. Before deposition, the substrates were ultrasonically cleaned in isopropanol at a frequency of 22 kHz for 5 min using the UZDN-2T installation (JSC «UKRROSPRIBOR», Ukraine). Additional cleaning via ion etching was performed right before the coating deposition using the slit-type ion source (Ar+ ions, 2 keV) for 20 min. The coatings were deposited under the following conditions: 80 mm distance between the substrate and the target, 10−<sup>3</sup> Pa residual pressure, and 0.1–0.2 Pa working pressure in the vacuum chamber. Ar (99.9995%) was used as the working gas, as well as its mixtures with N2 (99,999%) and C2H4 (99.95%). The gas flow rate (Table 1) was controlled by the gas supply system ELTOCHPRIBOR, LTD (Moscow, Russia). The constant current (1 kW power) was supplied to the magnetron using Pinnacle+ source (Advanced Energy, Fort Collins, CO, USA) during the whole deposition duration (40 min). Experiments were conducted without applying a bias voltage to the substrate. The schematics of the installation are given in the work [32].


**Table 1.** Composition, thickness, and deposition rate of coatings.

The elemental composition and structure of coatings were studied using scanning electron microscopy (SEM) using the S-3400 microscope (Hitachi, Tokyo, Japan) equipped with the energy dispersive spectroscopy add-on (EDS) Noran-7 Thermo. X-ray diffraction analysis (XRD) was performed on a D2 Phaser Bruker diffractometer using CuKα radiation. Elemental distribution profiles across the thickness of the coating were studied by glow discharge optical emission spectroscopy (GDOES) using Profiler 2 installation ("Horiba Jobin Yvon", Longjumeau, France). Studies of the fine structure were performed using a high-resolution transmission electron microscope (HR TEM) JEM-2100 Jeol. The foils for HR TEM were prepared using the ion-beam etching unit PIPS II System (Gatan, Pleasanton, CA, USA) and FIB (FEI Quanta 200 3D FIB instrument, Hillsboro, OR, USA). The coatings' mechanical properties were tested at a load of 2 mN on a nano-hardness tester (CSM Instruments, Peuseux, Switzerland) equipped with a Berkovich indenter. The tribological performance of the coatings was tested on an automated friction machine Tribometer (CSM Instruments) using a «pin-on-disc» scheme and a normal load of 1 N. Balls (d = 6 mm) made of alumina (hardness 19 GPa) and 100Cr6 grade steel (hardness of 8 GPa) were used as the counterparts for tribological experiments. Wear tracks were studied using an optical profiler Wyko-1100NT (Veeco, Plainview, NY, USA). To assess the coatings oxidation resistance, step-by-step annealing was performed in the air in a muffle furnace SNOL 7.2/1200 (Umega, Ukmerge, Lithuania) with a step of 200 ◦C, a maximum temperature of 1200 ◦C, and 1 h exposure. The specimens were inserted into a furnace heated to a given temperature T, dwelled during 1 h in the air, and afterward ejected from the furnace and cooled to 20 ◦C. The temperature was increased using 200 ◦C steps, and 1000 ◦C was the highest temperature the coatings could endure for 1 h without complete oxidation. At 1200 ◦C, all coatings were completely oxidized after 1 h long experiment; therefore, the oxidation duration for the test at this temperature was decreased to 10 min. Annealed coatings were studied by SEM and EDS. A more detailed description of the employed methods and equipment can be found elsewhere [33].
